Difficulties are encountered in practice in the treatment of contaminated water flows such as sewage prior to fine filtration, due to the presence of heavy and generally untreatable objects such as rags, string, plastic bags, and the like. Wastewater treatment plants typically utilize some type of screening equipment to remove harmful debris contained in the waste stream flow. Screening equipment is often utilized in the headworks section of the plant, and is the first area to come in contact with the waste stream. The screens are typically made from corrosion resistant materials such as 304 or 316 stainless steel, plastics or other synthetic materials. In order to protect the downstream equipment and processes, screening equipment is designed and incorporated in a plant to remove a large majority of debris before it comes into contact with any downstream equipment. If such items are not removed, proper and adequate treatment of the liquid does not result, and blockage of ducts and channels may occur.
There are many different screening equipment designs. A screening device is known comprising a continuously movable endless conveyor loop formed of a series of interconnected link pieces each having a lifting hook on which material to be screened is collected during movement through the contaminated water flow. Fine screens of this nature are typically defined by the size of the screen openings, which can be from as small as a quarter inch to one-half inch. These units can also be sized to have clear openings as large as 2 inches or more, but typically are not. The screen openings are designed to address both the horizontal and vertical limiting dimensions. The horizontal dimension is the small dimension and definition of the continuous belt, for example, one-quarter inch. The vertical dimension is typically significantly larger (approx. 4 in. or 6 in.) and is tied to the length of individual elements and the interconnected driving links and support shafts or pivot rods.
Another type of screening device is a panel type filter screen assembly. The panel type filter screen units generally include a plurality of filter panels and an endless chain operationally connected to the filter panels to move the filter panels through the water or wastewater while following a guided path in the structural frame of the filter screen. Typical panel type filter screen units utilize a plurality of filter panels that are fabricated of metal such as steel or stainless steel or other non-corrosive material. Such a panel typically consists of a large number of metal bars 2 spaced from and generally parallel to one another and welded vertically to a metal panel frame 4. The filter panel assemblies are operationally attached to the drive system, such as to an endless chain, and movement of the chain in turn moves the filter panel assemblies along a guided path within the structural frame of the screen filter apparatus 10.
An illustrative screen filter apparatus 10 is shown in FIGS. 1–3. Referring first to FIG. 1, it will there be seen that a waste material filtering apparatus 10 of the prior art generally includes a frame 11, a plurality of pipe spreaders 13, a drive motor 15 connected to a drive sprocket assembly 22 by a drive belt or chain 23, shown in phantom lines, and a rotating screen assembly 17 driven by the sprocket assembly. Referring to FIG. 2, the apparatus 10 sits in a channel 21 within which flows a stream of water containing solid waste.
The rotating grid assembly 17 is a link type grid that includes a plurality of vertically disposed, laterally spaced apart link members 12 that are disposed in articulated relation to one another. The trailing end of each link 12 has an integral horizontally-extending part 16 that helps hold and lift solid matter from the stream as the screen segments travel upwardly on the upstream side of the machine. Means are provided at the discharge end of the apparatus for dumping the matter so lifted into a solid waste collection container.
The opposite ends of each link 12 is mounted on a shaft 31, 33. The trailing end of each link 12 is the aforementioned horizontally-extending member 16 that helps hold and lift solid matter from the water stream as the links rotate, as is perhaps best understood by observing the links at the lower left corner of FIG. 1. The uppermost or leading end of each link is denoted 18. Plural directional arrows, collectively denoted 19, show the path of travel followed by the links as the machine operates. The orientation of machine 10 in a channel of water is shown in FIG. 2. The concrete channel is denoted 21. In this particular example, there are about twenty five upstanding screen segments disposed in equidistantly spaced lateral relation to one another, each screen segment being formed by a group of articulated link members 12.
The conventional assembly pattern of links is best understood in connection with FIG. 3. The links 12 of the prior art rotating screen are typically assembled in the following pattern. The trailing and leading ends 16, 18, respectively, of a link 12a are slipped onto a pair of contiguous shafts 31, 33, with the same procedure repeated about the remaining alternating shafts 31, 33 of the screen assembly 17. The trailing end of the next link 12b is then slid onto shaft 31 and the leading end of that link is slid onto shaft 33. The alternating pattern is then followed as links are placed on all of the shafts 31, 33. Spacers 20 are then added to each shaft, and the same pattern of assembly is repeated to construct a screen assembly 17 of a desired width. The alternating nature of the links 12a, 12b connects the links into a continuous loop.
The rotating grid assembly 17 is generally a large structure that is directly driven by the sprocket drive assembly 22 and is under significant tension. The sprocket assembly 22 generally includes a plurality of laterally spaced apart motor-driven sprocket segments. The sprocket segments are positioned in offset relation to the individual links 12 so that as the links 12 pass there over, the sprocket segments enter into the spaces between the links 12. Such a sprocket drive assembly 22 is subject to jamming because the apparatus performs well only when the sprocket segments are perfectly or almost perfectly aligned with respect to each contiguous set of links. In order to establish such perfect alignment, the spacers between the sprockets must be manufactured to very tight tolerances. However, a single drive shaft may include from 10 to 150 or more spacers. Any inaccuracy in the individual spacer tolerances is accumulated across the shaft to create a total inaccuracy that is often unacceptable. Even when the sprocket segments and links 12 are in their respective ideal relative positions, the sprocket segments rub against their contiguous links 12 in alternating succession, displacing each link about one-sixteenth of an inch per rub. More particularly, the links are displaced in a first direction in a first rub, and are displaced in an opposite direction during a second rub. Due to the rubbing and alternating displacement of the links, the sprocket segments and links 12 eventually become misaligned and collide with one another. This bends the links 12, damages the sprocket segments, and prevents further rotation of the rotating screens until the apparatus has been disassembled and new links 12 and sprocket segments installed. The repair procedure typically requires the grid assembly to be dismantled to expose the drive shaft and sprockets for repair or the entire assembly must be removed and returned to a shop for repair.
To define the path of the screen assembly 17, the prior art system includes sliding wear bars 40 to define the path of the screen loop adjacent the top of the filter assembly 10 and inner and outer guide rails 42, 44 and a bottom guide 46 are provided adjacent the bottom of the assembly 10 to track the screen assembly 17. The wear bars 40 are subject to significant wear from the continuous travel of the links 12 there over. The internal positioning of the wear bars 40 make them difficult to maintain and replace. Additionally, the wear bars 40 cause wear and additional frictional load on the screen assembly 17. The guide rails 42, 44 and bottom guide 46 are also subject to wear. Additionally, since these components 42–46 are in the fluid stream, they act as a point of debris build-up. The built up debris negatively effects flow throughput. Additionally, grit, sand and the like trapped between the guides 42, 44 and 46 and the screen assembly 17 acts to accelerate wear on the screen assembly 17. The limited accessibility makes component replacement and debris flushing difficult.